The objectives are 1) to determine the role of vitamin D in adaptation of calcium metabolism in the lactating rat with particular regard to interactions with parathyroid hormone (PTH) and calcitonin; and 2) to determine the effects of vitamin D status of the suckling pups on calcium and bone metabolism and secretion of calcitonin and parathyroid hormone. The role of vitamin D in the lactating rat will be determined a) from the sequential changes in the serum levels of calcium, 1,25-(OH)2D, parathyroid hormone and calcitonin in early lactation; b) from the effects of injections of 1,25-(OH)2D3 on the serum levels of calcium and phosphate and on intestinal calcium absorption (in vivo and in vitro) in vitamin D-deprived, hypocalcemic lactating rats; c) from the effects of consumption of diets with different vitamin D contents on the serum levels of calcium, phosphate and 25-OH2D3 in lactating rats and their pups; and d) from the effects of dietary vitamin D status on the rate of secretion of calcitonin and parathyroid hormone in vivo and from thyro-parathyroid glands incubated in vitro. The interactions of vitamin D, parathyroid hormone and calcitonin in the suckling rat pup will be determined with regard to parathyroid hormone secretion, bone metabolism and serum levels of calcium, phosphate, and magnesium.